The present invention relates to improvements in the field of borehold extensometry. More particularly, the invention is concerned with an improved borehole extensometer for monitoring relative mass displacements.
Borehole extensometers are instruments generally used to measure rock movements that may take place as a result of surface and underground excavation, foundation loading, movement of natural slopes, etc. The relative displacements of the rock are measured at various depths in a borehole and these displacement measurements allow to determine the relative stability of the rock surrounding the opening, to define those zones which are being compressed or put in tension and to possibly delineate the destressed volume of rock created by the excavation itself.
A borehold extensometer typically comprises three main components: a displacement sensor, one or more anchors at various depths in the borehole and rods or wires each interconnecting a respective anchor with the displacement sensor. The displacement sensor is usually mounted inside a reference head located at the borehole collar and can be designed for either electrical or mechanical readout. The relative displacements of the rock, as transmitted to the rods or wires, are measured by the displacement sensor as a change in the distance between an anchor within the borehole and the reference head at the borehole collar.
A borehole extensometer consisting of a single anchor and a rod or wire extending between the anchor and the reference head is termed "single-position borehole extensometer". Extensometers with a plurality of anchors within the rock are termed "multiple-position borehole extensometers" and are preferred for measuring rock movements since the variation of displacement with depth can be determined.
As the displacement sensor is located in a head assembly protruding from the collar of the borehole, it can be easily damaged, for instance by moving trucks, especially in mining areas. Even if fixed by rock bolts or grout at the borehole collar, the extensometer head is generally not sufficiently stable to blast vibrations and can thus be displaced by violent shock waves created by blasting, resulting in erroneous measurements.
Moreover, since a rod or wire extending between each anchor within the borehole and the reference head at the borehole collar is used to transmit to the sensor the relative displacement of the anchor caused by rock movement, long rod- or wire-type installations usally suffer from a lack of instrument precision due to unavoidable friction in such installations. In long vertical holes, buckling of the rods and excessive weight may also cause problems.